1. Field of the Invention
This invention relates to thermal barrier coating systems for airfoils used in the turbine portions of gas turbine engines, and specifically to an improved coating arrangement for segments consisting of a plurality of airfoils.
2. Discussion of the Prior Art
Airfoils, typically blades and vanes, used in the turbine portions of gas turbine engines, such as jet engines, are typically made from superalloy materials. These materials are utilized because of their high temperature properties and the ability to withstand the corrosion and oxidation resulting from the combustion exhaust typically occurring in such engines. As the temperatures have been driven constantly higher, it has been necessary to not only modify the compositions of these materials, but also it has been necessary to add thermal barrier coatings and cooling channels to allow continued performance.
Typically, these airfoils operate under conditions of stress at very high temperatures, so that not only are oxidation and corrosion a concern, but fatigue, particularly low cycle fatigue, is also a concern. These stresses can be caused by the operating conditions experienced by the part, or may be inherent in the mechanical design and/or fabrication of the part and can shorten the operating life of an article. Fortunately, these conditions occur over an extended period of time and provide ample warning signs that a problem exists. Nevertheless, it is desirable to identify the sources of fatigue whenever possible, so that the problems due to fatigue can be reduced or eliminated. One effective way addressing the problem of fatigue is to reduce stresses in the part. If the stresses can be reduced below a critical threshold value, fatigue can be eliminated. However, even if the stresses cannot be reduced below the critical threshold value, any reduction in stresses can result in an increased life for the part.
Airfoils such as turbine blades in the hot turbine section of a jet engine have shown early signs of distress in a region near the tip of the airfoil. These blades are coated with a thermal barrier coating along the tip of the airfoil, in the region where the airfoil interfaces with the shroud assembly and along the leading edge of the airfoil from the tip of the airfoil to the base. The thermal barrier coating is applied to improve the thermal performance of the blade, allowing it to operate at higher temperatures. Because the high temperature environment and stresses resulting from it cannot be changed and will only be modified toward increasing temperatures, it would be advantageous to reduce the stresses encountered in this region of the blade to eliminate or to delay the onset of these early signs of distress.
Improvements in manufacturing technology and materials are the keys to increased performance and reduced costs for many articles. As an example, continuing and often interrelated improvements in processes and materials have resulted in major increases in the performance of aircraft gas turbine engines. The present invention is an improvement in manufacturing technology in the application of materials to bring about improved performance of airfoils in the turbine portion of a gas turbine engine.